gcc-7.5.0 does not build anymore due to #1595

[wwwutz]

mpfr update ( #1595 ) needs to have a new gmp.h

configure:5564: checking for the correct version of gmp.h
configure:5584: gcc -c -g -O2   conftest.c >&5
configure:5584: $? = 0
configure:5602: gcc -c -g -O2   conftest.c >&5
configure:5602: $? = 0
configure:5603: result: yes
configure:5619: checking for the correct version of mpfr.h
configure:5637: gcc -c -g -O2   conftest.c >&5
configure:5637: $? = 0
configure:5654: gcc -c -g -O2   conftest.c >&5
configure:5654: $? = 0
configure:5655: result: yes
configure:5672: checking for the correct version of mpc.h
configure:5689: gcc -c -g -O2   conftest.c >&5
In file included from conftest.c:10:0:
/usr/include/mpc.h:107:9: error: unknown type name '__gmp_const'
 typedef __gmp_const __mpc_struct *mpc_srcptr;
         ^~~~~~~~~~~
/usr/include/mpc.h:107:34: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
 typedef __gmp_const __mpc_struct *mpc_srcptr;
                                  ^

[wwwutz]

fixed by #1605